The present invention concerns an optical element made of material that is transparent to light from at least a first and a second spectral band, where in the element an optical system is embedded which comprises:
a first optical layer system in a first plane which at least predominantly reflects light from the first band and at least predominantly transmits light from the second band; PA1 a second optical layer system in a second plane that intersects the first plane in a central area of the body and at least predominantly transmits the light from the first band and at least predominantly reflects the light from the second band; PA1 with a first entrance or exit surface for light from the first band; PA1 with a second entrance or exit surface for light form the second band, as well as PA1 with an exit or entrance surface for light from the first as well as the second band. PA1 U.S. Pat. No. 2,737,076 PA1 U.S. Pat. No. 2,754,718 PA1 DE 40 33 842 PA1 JP 7-109443 PA1 U.S. Pat. No. 5,098,183 PA1 EP 0 359 461 PA1 .DELTA.l.sub.0 : is the difference of the optical length of path l.sub.01, l.sub.02 for light L.sub.1, L.sub.2 of the two mentioned spectra; PA1 n.sub.1, n.sub.2 : are the refractive indices of the material of the optical element along the corresponding optical paths; PA1 W: is the geometric path of light L.sub.1, L.sub.2. PA1 a: Object distance=AK.sub.3 PA1 a' Image distance=K.sub.1 A' PA1 f': Focal length of the system PA1 f'=50 mm, PA1 a=-60 mm PA1 .DELTA.l=-0.4784 mm,
The present invention is based on problems that result from so-called X-cubes. An X-cube is a specific design version of said optical element and is a preferred design version also within the framework of this invention. On the X-cube the two planes of said optical layer system intersect perpendicularly. The optical element, intersected perpendicularly to the plane of the layer system, defines a square surface. The two entrance or exit surfaces are formed by parallel, plane surfaces as well as the exit or entrance surface that is perpendicular thereto. Opposite the latter is another entrance or exit surface, a third one, is provided for light from a third spectral band.
Such elements which are often referred to as X-cubes, a term that is also used in the present description, are used, for example, in projectors in order to recombine RED/GREEN and BLUE channels. Rather than for recombination such elements can also be used for color splitting by reversing the optical path. With respect to such elements, reference can be made to
and with respect to their application, to
We also refer to the PCT application PCT/CH97/00411 corresponding to U.S. patent application Ser. No. 08/756,140, filed Nov. 26, 1996 which is an integral part of this description and explains in particular the preferred manufacturing process for such X-cubes or optical elements.
FIG. 1 illustrates the utilization of an X-cube for light recombination. The light L.sub.1 from a first spectral range, such as in particular the RED range of EQU 600 nm to 800 nm
is reflected via the first entrance or exit surface K.sub.1 on the first layer system S.sub.1, partially after transmission by the second optical layer system S.sub.2, and leaves the X-cube at the exit or entrance surface K.sub.4.
Light L.sub.2 from a second spectral range, in particular the BLUE range of EQU 400 nm to 500 nm
is applied to the second entrance or exit surface K.sub.2, is reflected--after partial transmission by the first system S.sub.1 --by the second optical layer system S.sub.2 and reaches said exit or entrance surface K.sub.4 together with light L.sub.1.
In particular when such an optical element, in particular an X-cube, is used in said application, light L.sub.3 from a third spectral range, in particular the GREEN range of EQU 500 nm to 600 nm
is applied to the optical element at a third entrance or exit surface K.sub.3, transmitted by both optical layer systems S.sub.1 and S.sub.2, and leaves the element at the common exit or entrance surface K.sub.4.
The optical layer systems S.sub.1 or S.sub.2 respectively are formed by one or more optically effective layer(s) as described in particular in U.S. patent application Ser. No. 08/756,140, filed Nov. 26, 1996, and incorporated here by reference.
The light L1, L2 and L3 from said three spectral ranges is often applied to element 3 via light valves l.sub.1, l.sub.2 and l.sub.3, in particular through LCD arrangements. By means of the optical layer systems S.sub.1 and S.sub.2, which are dielectric reflectors, the imaginary images of the first and second spectral range, corresponding to L.sub.1 and L.sub.2, and as mentioned, in particular of the RED and BLUE spectral range, are made to coincide via the corresponding light valves l.sub.1 and l.sub.2 with the real image of the third spectrum, corresponding to L.sub.3, particularly preferred from the GREEN spectrum, via the assigned light valve l.sub.3.
The better the coincidence the better the projected image.
This requires in particular very close mechanical tolerances for the optical element.
As mentioned, such an element can be used for color splitting by reversing the optical path illustrated in FIG. 1, in particularly for use with a CCD camera. Such an element--not only in the specific design as an X-cube but also in the generalized form referred to at the beginning--is in particular afflicted by the disadvantages explained below.
If, as shown schematically in FIG. 2 with respect to the special situation of the X-cube, light L.sub.1 from the first spectral range and light L.sub.2 from the second spectral range impinge on element 3, this normally results in a difference in the length of optical path l.sub.01, l.sub.02 between the corresponding entrance and exit surfaces K.sub.1 and K.sub.2 respectively, the assigned optical layer systems S.sub.1, S.sub.2 and the exit or entrance surface K.sub.4, due to dispersion even if along the optical paths of light L.sub.1 and light L.sub.2 the same component material is used which in the case of X-cubes is normally a glass, in particularly BK7 glass. For the illustrated perpendicular light incidence the difference in the length of path is as follows: EQU .DELTA.l.sub.0 =n.sub.2 -n.sub.1 (W)=l.sub.02 -l.sub.01
where
If along the optical paths different geometric paths W.sub.1 or W.sub.2 must be taken into consideration, the formula is: EQU .DELTA.l.sub.0 =n.sub.2 .cndot.W.sub.2 -n.sub.1 .cndot.W.sub.1
Normally, however, identical mechanical or geometric lengths of path W.sub.1, W.sub.2 are chosen. For an X-cube made of BK7 glass with a cross-sectional edge length of 40 mm we obtain for light of a given wavelength L.sub.1 in the first spectral range: EQU .lambda..sub.1 =643.8 nm
and for light from a second spectral range of a given wavelength .lambda..sub.2 : EQU .lambda..sub.2 =435.8 nm
an optical path length difference .DELTA.l.sub.0 of EQU .DELTA.l.sub.0 =0.4789 nm=40 nm.cndot.(n.sub.2 -n.sub.1).
For an imaging system downstream the optical element produces an image in the spectrum corresponding to L.sub.1 and in the spectrum corresponding to L.sub.2, in particular in the RED and BLUE spectra, with simultaneous light impingement on the element, which results from two object planes that are mutually offset by .DELTA.l.sub.0.
This difference in the optical path lengths can be corrected externally with respect to the optical element by installing the light valves l.sub.1 and l.sub.2 shown in FIG. 1. at different distances from the corresponding entrance and exit surfaces K.sub.1 and K.sub.2 respectively. However, this requires a considerable installation and adjustment effort. If light valves l.sub.1 and l.sub.2 are installed equidistant from the assigned entrance or exit surfaces of the element, this leads to image blurring due to said differences in the optical path lengths.
The imaging equations for an optical system with the main planes K.sub.1 and K.sub.3 as shown in FIG. 3 and exposed to air on both sides give EQU l/a'=l/a=1/f'
where
The difference of the object distance by .DELTA.l causes a difference of the image distance by .DELTA.a' resulting in: EQU .DELTA.a'=f' (a+.DELTA.l)/(f'+a+.DELTA.l)-f'a/(f'+a).
This means that also the imaging scale .beta. is influenced. For the latter the formula is: EQU .beta.=a'/a.
For an image that should have a magnifying effect, object A must be within the single to double focal length f', that is,: EQU f'.ltoreq..vertline.a.vertline..ltoreq.2f'.